The present invention is related to the field of optical fiber technology and, more particularly, to fiberoptic couplers, especially wavelength division multiplexed (WDM) couplers, and fiberoptic isolator devices.
In strict fiberoptic terminology, fiberoptic "couplers" are devices which transfer the light signals from a plurality of input fibers to a plurality of output fibers. "Combiners" are devices by which the light signals from more than one input fiber are combined into a single output fiber. However, as explained below, the present invention is readily adaptable to both types of devices. Hence the term, "coupler," as used with respect to the devices of the present invention is meant to be interpreted broadly and to cover both terms.
A WDM coupler transfers input signals from a plurality of input information channels to a plurality of output information channels in response to the wavelength of the input signals. A goal for any WDM coupler is that the crosstalk between channels is zero, i.e., that an untargeted output channel is effectively isolated from the signals on a targeted output channel so that none of the signals leak onto the untargeted channel.
FIG. 1A is a representational diagram of a 2.times.2 WDM coupler found in the prior art. The cladding and core of a pair of optical fibers are fused together to form a WDM coupler 10 enclosed by dotted lines. The coupler 10 has two input fibers 11, 12 and two output fibers 13, 14. The first input fiber 11 carries signals of wavelength .lambda..sub.1 and the second input fiber 12 carries signals of wavelength .lambda..sub.2. Ideally, only one of the output fibers, say output fiber 13, should carry the signals of wavelength .lambda..sub.1, while the other output fiber 14 should carry the signals of wavelength .lambda..sub.2, as shown in FIG. 1A. Crosstalk occurs if the .lambda..sub.1 signals appear on the output fiber 14 or the .lambda..sub.2 signals appear on the output fiber 13.
An application of the fused coupler of FIG. 1A is a partially integrated WDM coupler and isolator device, which is described in U.S. Pat. No. 5,082,343, issued Jan. 21, 1992 to D. G. Coult et al. The fused WDM coupler is illustrated in FIG. 1B. The coupler 20 (again enclosed by dotted lines) has two input fibers 15, 16 and an output fiber 17. The two input fibers 15 and 16 are fused together and signals from the fused fibers are directed toward a collimator 18. A second collimator 19 refocuses the collimated light signals from the collimator 78 into the output fiber 17. The collimators 18 and 19 are shown as standard lens for purposes of illustration. One input fiber 15 carries signals of wavelength .lambda..sub.1 ; the second input fiber 16 carries a signals of wavelength .lambda..sub.2. A wavelength selective element 21 between the collimators 18 and 19 reflects the light of one of the wavelengths, say .lambda..sub.1, and passes the .lambda..sub.2 wavelength light. Thus the output fiber 17 carries the .lambda..sub.2 signals.
The problem with this WDM coupler and isolator arrangement is the crosstalk between the fibers 15 and 16 carrying the reflected .lambda..sub.1 signals. As explained below, the .lambda..sub.1 signal should ideally be reflected into the input fiber 16 only. In reality, some of the .lambda..sub.1 signal is reflected back into the input fiber 15; there is crosstalk. Besides crosstalk, another problem is that the insertion losses and polarization dependent losses of such couplers are high. Additionally, the device is rather large, which has an adverse effect upon the reliability and robustness of the device. Sealing the device, for example, is more of a problem with a large device. The large size also makes the device difficult to insert into various points of a fiberoptic network system, as may be desired. Still another disadvantage of the described device is that other desirable components, such as a tap coupler for monitoring signals through the device, must still be linked by fiber splicing. This lowers the performance of the overall system and creates further reliability problems.
In contrast, the present invention avoids, or substantially mitigates, the problems of the fused coupler. A fiberoptic coupler according to the present invention has a much higher optical performance and is easily integrated with other optical elements to create integrated couplers and optical isolators with advanced features and performance. Among these coupler and isolator devices is included an integrated WDM coupler and isolator device with high isolation between channels. The device is a great improvement over the WDM coupler and isolator described in the patent noted above.
Furthermore, these advanced couplers and isolators provide for advanced fiberoptic systems of higher performance, lower cost, and superior reliability.